1. Field of the Invention
The present invention is directed to a standing wave barrier for a conductor having a conductor section in which a standing wave would be induced by a high-frequency field without the standing wave barrier, of the type having at least one basic wave trap and a capacitor.
2. Description of the Prior Art
Standing wave barriers are well known. They are particularly utilized in magnetic resonance systems in order to prevent circularly or linearly polarized high-frequency magnetic fields from inducing high-frequency standing waves in the outside jacket of coaxial cables. In this application, thus, the outside jacket of the coaxial cable represents the conductor or the conductor section.
In known standing wave barriers, the basic wave trap is galvanically connected to the outside jacket and to a capacitor. The capacitor is also galvanically connected to the outside jacket. The basic wave trap surrounds the outside jacket over a part of its length.
Standing wave barriers of this type exhibit a number of disadvantages. In particular, that cannot be balanced until after mounting at the coaxial cable. Moreover, when a number of such-standing wave barriers are arranged on a single cable, they mutually influence each other. The balancing becomes more complicated as a result.
Further, guide wires of metal are known, particularly for catheters. These are thin wires that can be introduced into the inside of the body of a human and with which an item slipped over the wire, for example a catheter, can be guided to a specific location in the human body. The location is defined on the basis of medical criteria. Standing waves also can be induced in such guide wires. Devices for preventing standing waves for guide wires are still unknown.
An object of the present invention is to provide a standing wave barrier with which the aforementioned disadvantages of known barriers are eliminated. In particular, it is an object to provide such a standing wave barrier that is also employable for guide wires.
The object is achieved in a standing wave barrier having an electrically conductive interior structure which surrounds a region of the conductor section in which a standing wave can arise, and in which the conductor section is displaceable. The interior structure is galvanically separated from this region of the conductor section, but is inductively coupleable thereto. The basic wave trap surrounds the interior structure over a part of its length and is galvanically connected to the interior structure and a capacitor. The capacitor is galvanically connected to the interior structure.
As a result of the inventive design, it is possible to assemble and balance the standing wave barrier before being connected to the conductor. In particular, a number of standing wave barriers can be balanced individually and independently of one another, i.e. without mutual influencing. Due to the displaceability of the inside structure on the conductor section, the standing wave barrier also can be utilized for a guide wire.
In the simplest case, the standing wave barrier has a single wave trap, referred to below as a basic wave trap.
alternatively, the standing wave barrier may have two wave traps. In the case of two wave traps, the terms basic wave trap and auxiliary wave trap are employed in order to distinguish the wave traps from one another. Optionally, the wave traps can be fashioned identically or differing from one another.
When the inside structure is fashioned as hollow cylinder with a cylinder diameter and a cylinder length and the cylinder length is considerably longer than the cylinder diameter, a good coupling of the standing wave barrier to the conductor section is achieved. In this case, in particular, only a transverse electromagnetic wave can form between the conductor section and the standing wave barrier, which promotes the coupling of the standing wave barrier to the conductor section.
In an embodiment, the capacitor is a variable capacitance, allowing the standing wave barrier to be balanced in an especially easy way.
In an embodiment, the inside structure is clad in an electrically insulating fashion at the inside, it can also be employed at guide wires even if the guide wire itself has no insulation.
In a further embodiment, a high-frequency voltage drop across the capacitor is supplied via a pre-amplifier to an evaluation circuit with which the position of the conductor can be determined. In this case the location of the conductor can be reconstructed, particularly in magnetic resonance systems, as long as it is in the imaging volume. This is particularly significant for the localization of a guide wire in the body of a human (or of an examination subject in general).
A separating circuit with which the pre-amplifier can be decoupled from the capacitor and be arranged between the capacitor and the pre-amplifier, so the pre-amplifier is protected against damage in a magnetic resonance system (which operates with high power) in a simple way.
If the inside structure and the basic wave trap, and the auxiliary wave trap as well, are flexibly fashioned, then the standing wave barrier can be utilized in especially versatile ways.